1. Field of the Invention
The present invention relates to a pupil center detection method, a pupil ring detection method, and an apparatus adopting these methods and, more particularly, to a method and apparatus suitably used in a case wherein a light beam is radiated from a projection means onto an eyeball of an observer (photographer), a first Purkinje image (a cornea reflecting image) and a pupil image based on light reflected by the eyeball are formed on an image sensor surface, and the sight axis of the eyeball is detected using the position coordinates of these images on the image sensor surface.
2. Related Background Art
Conventionally, there have been proposed various sight axis detection apparatuses for detecting the observation position of an observer (photographer) on an observation surface, i.e., detecting a so-called sight axis of the observer.
For example, in Japanese Laid-Open Patent Application No. 61-172552, a parallel light beam from a light source is projected onto a front eye part of an eye to be detected, and the sight axis (view point) is obtained by utilizing a cornea reflecting image based on light reflected by a cornea and a focusing state at a position of the pupil center.
FIG. 18 is an explanatory view of a sight axis detection method according to an embodiment proposed by the above patent. FIGS. 19A and 19B are respectively a front view of an eyeball shown in FIG. 18, and a chart showing an output signal from a line sensor shown in FIG. 18.
In FIG. 18, a light source 704 such as a light-emitting diode for emitting infrared light unsensible by an observer is arranged on a focal plane of a projection lens 706. Infrared light emitted from the light source 704 is collimated into parallel light by the projection lens 706, is reflected by a half mirror 710, and illuminates a cornea 701 of an eyeball 700. At this time, some light components (cornea reflecting image or Purkinje image) of the infrared light reflected by the surface of the cornea 701 are transmitted through the half mirror 710, are converged by a light-receiving lens 707, and form an image at a position d' on an image sensor 709.
On the other hand, light beams reflected by edge portions (pupil ring portions) a and b of an iris 703 are guided onto the image sensor 709 via the half mirror 710 and the light-receiving lens 707, and form images of the edge portions (pupil ring portions) a and b at positions a' and b' on the image sensor 709. When a rotational angle .theta. of an optical axis T of the eyeball with respect to an optical axis S of the light-receiving lens 707 is small, if the z-coordinates of the edge portions a and b of the iris 703 are represented by Za and Zb, a coordinate Zc of a central position (pupil center) position c of the iris 703 is given by: ##EQU1##
If the z-coordinate of the generation position d of the cornea reflection image is represented by Zd, and the distance between the center O of curvature of the cornea 701 and the center C of the iris 703 is represented by LOC, the rotational angle .theta. of the optical axis T of the eyeball substantially satisfies: EQU LOC.multidot.sin .theta..congruent.Zc-Zd (1)
For this reason, the rotational angle .theta. of the optical axis T of the eyeball is obtained by detecting the positions of specific points (images Zd', Za', and Zb', on the image sensor 709, of the generation position d of the cornea reflecting image and edge portions a and b of the iris) projected onto the image sensor 709, thereby obtaining the sight axis of the observer. At this time, relation (1) can be rewritten as: ##EQU2## where .beta. is the magnification determined by a distance L between the generation position d of the cornea reflecting image and the light-receiving lens 707, and a distance L.sub.0 between the light-receiving lens 707 and the image sensor 709, and normally assumes an almost constant value.
As described above, when the sight axis direction (view point) of the eye to be detected of the observer is detected, the observation position on the focusing screen can be known in, e.g., a single-lens reflex camera.
FIG. 19B shows the brightness distribution of an image on a 1-line sensor array 709a of the image sensor 709. Since the iris 703 has a higher reflectance than that of a pupil 711, the image of the iris 703 becomes brighter than that of the pupil 711. In FIG. 19B, points Ra and Rb where the brightness largely changes correspond to two points a' and b' where the boundaries between the pupil 711 and the iris 703 cross the sensor array 709a.
Note that Rc is an output corresponding to a Purkinje effect image. For example, digital data converted by an A/D converter in units of pixels are checked from the two ends of the sensor array 709a to obtain points where the output changes from bright level La to dark level Lb for the first time, thus obtaining the two points a' and b' where boundaries a and b between the pupil 711 and the iris 703 cross the sensor array 709a. From these two crossing points a' and b', the pupil center can be obtained by averaging the pupil ring portions a and b and the two points a' and b'.